Study On The Response Of Drainage Law Of Subsurface Drainage To Irrigation Process Under Mulch Drip Irrigation

Soil salinization is a global concern of agricultural soil environmental problems,but also second only to desertification of the ecological environment,its improvement and control has been favored by scholars.In recent years,the underground drainage technology has been proved by most scholars to be one of the most effective and direct hydraulic engineering measures to improve saline-alkali land.In Xinjiang,where nearly one third of the cultivated land was damaged by soil salinization,the improvement and control of saline-alkali cultivated land is an urgent problem to be solved and concerned about.Based on the condition of shallow groundwater burial,the effects of different buried depth of underground pipe and irrigation amount on the drainage and salt discharge of underground pipe and soil desalination efficiency were explored by soil column test.The influence of irrigation process on the drainage and desalination law of underground pipe and the effect of soil desalination were discussed by soil tank experiment.The transport rules of soil water and salt during irrigation and drainage were simulated and verified by using HYDRUS model.It has a good reference value for the determination of technical parameters of subsurface drainage in saline farmland in arid area and the formulation of scientific and reasonable irrigation and leaching system.We can obtain some primary conclusions are as follows:(1)Irrigation amount had obvious influence on the distribution and movement of soil water and salt in different soil layers.The change rate of water and salt in 0-20 cm soil layer was faster than that in 20-100cm soil layer,where the water-salt content in the vertical direction increased gradually.The smaller the irrigation quantity was,the greater the salt accumulation degree in the lower soil layer was.The larger the irrigation quantity was,and the smaller the salt accumulation degree in the soil layer under the underground pipe was,and the upper desalination efficiency was higher than the lower.The salinity,drainage volume,salt discharge and salt content of underground water and groundwater increased with the increase of irrigation quantity.But when the irrigation quantity was the maximum,the salt content of groundwater and groundwater was much larger than that of drainage and salt discharge,which indicated that the excessive water will not only raise the groundwater level,but also make the salt content of upper soil drain into groundwater.(2)Salt content in each soil layer increased by soil depth increasing in three kinds of buried depth.At the same irrigation amount,the deeper the buried depth,the less the content of water and salt in the same soil layer,and vice versa.The deeper the buried pipe is,the larger the drainage volume and salt discharge of the underground pipe are;the smaller the groundwater increment is,and the salt content of the groundwater increased first and then decreased by the irrigation quality aggrandizing.The maximum desalination rate was 90%in 0-20 cm soil layer,78%in 0-100 cm soil layer and 42%in minimum.There was no obvious difference in desalination rate among different soil layers under the same amount of irrigation and different buried depth of underground pipe,but also obvious difference under the same buried depth of underground pipe and different amount of irrigation.It showed that the efficiency of drainage and salt discharge of underground pipe were mainly affected by the buried depth of underground pipe,while the effect of irrigation content on soil desalination efficiency of underground pipe was greater.(3)Under the condition of shallow groundwater level,we conducted an experiment with underground pipe drainage system of 1 m deep 4 m spacing pipes.After six times of irrigation,the water and salt content in 0-60 cm soil layer changed rapidly,the salt content decreased to 4 g/kg,the water and salt content in60-100 cm soil layer changes slightly,and the salt content decreased to about 6 g/kg.The larger the distance between underground pipes,the smaller the desalination rate,vice versa.The desalination rate was the highest in 0-20 cm soil layer and the lowest in 60-80 cm soil layer.According to the calculation,the salt content in 0-80 cm soil layer decreased by 72.9%as a whole,and the amount of salt discharged by underground pipes accounted for 30.8%of the total salt content in 0-100 cm soil layer.The rest of the salt was leached into the soil layer below 100 cm or dissolved into groundwater.The larger the distance between horizontal and underground pipes,the smaller the soil desalination rate,and the smaller the distance between horizontal and underground pipes,the larger the desalination rate.The variation of drainage salinity with irrigation times was basically stable in the range of 4-6 g/L,and the drainage velocity was stable in the range of 0-4 L/h.The transport law of soil water and salt were simulated and verified by HYDRUS model under the condition of underground pipe drainage.The root mean square error RMSE of soil salinity and moisture was 0.632 and 1.324 respectively,and the determination coefficient R~2 was 0.992and 0.906 respectively.The results showed that the simulated values were in good agreement with the measured values,and the HYDRUS model can simulate the soil water and salt transport law under the condition of drainage better.